The invention relates generally to photographic still cameras having a housing provided with a laterally arranged cassette receptacle into which an exchangeable cassette unit assembled of a main cassette part and a detachable cassette part can be inserted whereby the main cassette part is held in position at the inlet of the cassette receptacle whereas the detachable cassette part is movable transversely to the optical axis by means of a sliding carriage movable behind a film framing platform in the camera housing and cooperating with a swingable film pressure plate.
The known still camera of this type employs exchangeable cassette units designed for a roll of film whereby upon insertion of the cassette unit into the cassette receptacle formed on the camera housing the detachable cassette part is engaged by an entrainment piece of a sliding carriage and by advancing the shutter cocking lever of the camera it is displaced past a film gate into its detached working position. In doing so, a certain length of the film strip is unwound from the film supply roll arranged in the cassette main part and this film length is stretched behind the film gate. The film gate is formed in a movable film pressure frame which, in order to permit the passage of the sliding carriage, is displaced forwardly in the direction of the camera objective and subsequently is returned to its initial position where it is depressed against a film pressure plate to hold the length of the film strip in the film plane. Upon the release of the shutter, the film pressure frame is again lifted from the film pressure plate to give way to the movement of the sliding carriage. The sliding carriage is simultaneously returned into its starting position in the cassette receptacle so that the detachable cassette part is reunited with the main cassette part. During this return movement, the film takeup spool which is arranged in the detachable cassette part is driven to rotate in the film feeding direction so that the length of the film which has been exposed during the preceding shutter release is wound on the film takeup spool. During the subsequent renewal of the picture shooting condition of the camera effected preferably by means of the shutter cocking lever, the aforedescribed process is repeated whereby during the travel of the sliding carriage toward its working position remote from the cassette receptacle, the rotation of the film takeup spool is arrested so that a new length of the film strip is unwound from the film supply spool in the main cassette part.
This known camera, due to its aforedescribed construction, is suitable exclusively for operation with roll films but is unsuitable for film strips of the picture format 24.times.36 mm, usually designated as small format films. In the case of roll films, the frame advance is indicated by marks such as numerals provided on the rear side of the film backing paper and thus the film length withdrawn from the film supply spool can be determined. In the aforedescribed known camera, the film framing platform formed by the film pressure frame and the film pressure plate is considerably offset in the direction of the incoming light behind the film deviation roller in the film cassette unit. In displacing the sliding carriage into its working position remote from the cassette receptacle, the film strip as mentioned above is withdrawn from the film supply spool in the main cassette part while the film takeup spool in the detachable cassette part is blocked. As a consequence, the film strip portion from the deviation roller to the takeup spool forms an acute relative to the optical axis in front of the film pressure plate. As a consequence, when the film pressure frame is applied against the film pressure plate, it can happen that an additional film section is withdrawn from the film supply spool when an exact planar position of the film at right angles to the optical axis of the film gate is being adjusted.
By contrast, in the case of small format films the picture frame advance is determined by a driven sprocket or perforation roller which engages with its sprockets into the perforations provided in the marginal portion of the small format film and, according to its predetermined number of rotations, withdraws a corresponding film length from the film supply spool. Provided that such a small format film be used in cassette units for the known camera, it would be necessary to replace the film deviation roller with the perforation roller. If, however, the sliding carriage should operate in the afaoredescribed manner, a corresponding drive would have to be provided for the perforation or sprocket roller in order to insure the intermittent advancement of the film frames. Even in this case, the film section stretched between the perforation or sprocket roller and the takeup spool would extend obliquely to the film pressure plate in the end position of the sliding carriage remote from the cassette receptacle. If, in this oblique position of the film strip, the film pressure frame is applied against the film pressure plate, the provision of a certain backlash would have to be made either in the perforation roller or in the film takeup spool in order to adjust the film in the film plane perpendicularly to the optical axis. The provision of such a backlash means not only a considerable increase in construction costs but also may cause inaccuracies in the positioning of the film section which in turn may cause, particularly during rapid succession of shots, that the consecutive exposed frames may overlap in their marginal zones.
Due to the above-described structural concepts of the known camera, it can be designed only as a viewer camera. Due to the spatial requirements necessary for the movement of the displaceable film pressure frame, it is impossible to design the conventional cameras of this type as single-lens reflex cameras. Furthermore, the prior art cameras do not exclude the possibility of an incorrect manipulation, for example, the removal of the cassette main part from the receptacle while the detachable cassette part is in its remote position and consequently considerable inconveniences may result.